Are men and women who dress immodestly urged toward promiscuity?
Webster's Dictionary defines immodest as: not modest; not decorous; indecent; not shy or humble; bold; forward.
Immodesty doesn't necessarily go hand in hand with promiscuity. It can lead to promiscuity, but it is not a given. A person can be fully clothed and still be immodest. Garish colors, too tight of a fit, or excessive amounts of jewels can cause one to be immodest. Paul addressed this in I Timothy 2:9-10, "In like manner also, that the women adorn themselves in modest apparel, with propriety and moderation, not with braided hair or gold or pears or costly clothing, but, which is proper for women professing godliness, with good works." What a godly woman should be know for is her good works and righteousness first, rather than for her pearls and gold studded hair.
Modesty is as much an attitude as what you wear. Many people can be clothed properly, yet their actions are immodest. They draw attention to themselves unnecessarily, or act in vulgar disgusting ways. This applies to men and women equally.
Sexual promiscuity comes from ignorance of or a disregard for God's laws and lust. People act that way they have learned or want to.
I hope this has answered you question.